Plants form a complex interface between the soil and the atmosphere, two spatially and temporally variable systems. In this role, plants mediate and respond to—in their growth potential, yield, and seed maturation—the mass and energy fluxes that are defined by these boundaries. Conventional agricultural and ecological assessments of the physical and chemical state of this coupled system depend on measurements of low temporal and spatial resolution in the soil and the atmosphere. Thus, the instantaneous state of the plants must be extrapolated from this indirect data, and, conversely, the role of the plants in defining their environment must be estimated via models. Furthermore, the behavior of individual plants or sub-populations within a heterogeneous environment must be deconvolved from global measurements.
Water potential (ψw) is a form of chemical potential of water (μ) that is the most widely measured plant water status parameter and is a useful indicator to predict plant and fruit growth, yield, and fruit composition and quality. One advantage of using water potential to quantify plant water status is that at equilibrium, both liquid and vapor (gas) phases have identical values of water potential. “Water potential” as used herein refers to the chemical potential of sap water (μw) relative to that of pure water (μ0) at the same temperature and atmospheric pressure, and is synonymous with “plant water status” or the “chemical potential” of the sap within the xylem. The relationship between water potential (ψw) as used by the plant scientific community and chemical potential (μ) as used by the general scientific community may be expressed as:
                              ψ          W                =                                            μ              W                        -                          μ              o                                                          V              W                        _                                              (        1        )            
where Vw  is the partial molal volume of water. For purposes of discussion herein, the term “chemical potential” will be used with the understanding that the term could also mean water potential.
Plant water relations are governed by chemical (e.g., water) potential and its gradients, not by water content. There have been many studies of the effects of water stress on plant productivity and product quality though these have not always led to predictive tools. The main problem is that in large woody plants such as trees or grapevine variations, in-water status are very dynamic, both daily and seasonally, as plant water stress responds to both soil moisture and atmospheric evaporative demand. However, the mechanisms are not yet fully understood. Part of the reason for this is that there are presently no effective tools available to easily quantify and continuously monitor plant water status. Ideal measures must be direct measurement of chemical potential, continuous to capture the dynamics, and inexpensive enough to allow spatial resolution.
Probing and determining the plant water status, or state of sap water, in xylem (the water-transporting vascular system) presents fundamental challenges. For example, during active transpiration, the xylem conduits of plants maintain an extreme physical environment. Under common physiological conditions the sap water is at large negative pressures, e.g., Pxylem<−10 atmospheres (atm.), due to extreme capillary pull generated in the leaves as evaporation occurs. The liquid is thermodynamically metastable with respect to its vapor. This metastability has been a major obstacle to the development of synthetic interfaces with xylem.
There is currently no known effective way to monitor the state of water sap continuously in the field. Soil moisture, an indirect measure, can be monitored with several methods and can be useful. However, due to the strong role of aerial evaporative demand and variable root distributions, indirect soil moisture measurement is often inadequate. Other methods have been tested in an effort to determine plant water status. For example, pressure chamber methods provide accurate measurement and allow for a direct estimate of chemical potential within the plant, but are manual, slow and do not provide continuous data. Electronic stem dendrometers provide continuous monitoring of the shrinking and swelling of the plant stem, but the relation to chemical potential is not consistent. Another method utilizes remote sensing of canopy temperature by IR thermometry or multispectral scanning, but these methods are technically complex to implement, expensive, indirect and normally only done very occasionally. Measurement of 13C/12C isotope ratios of carbon in plant tissues can be a useful integrator of water stress at over extended periods, but not for monitoring real-time. A “stem psychrometer” has been used in research to directly monitor plant water status in the stem of a plant. A stem psychrometer has a small chamber with a thermocouple junction that is cooled to reach the dewpoint of the enclosed air. The stem version has one side of the chamber open and it is sealed against the xylem of a plant to make a tight chamber. With extremely accurate measurement and control of temperature, the stem psychrometer can estimate the chemical potential at a point in the tissue. These have been used successfully by a few scientists but they are very difficult to use, are often unstable and give many artifacts. The Agricultural Electronics Corp. has manufactured the PHYTOGRAM™ sensor, comprising a metal wire embedded inside the plant tissue. The manufacturer claims to estimate the tissue hydration or water content by measuring protons in the spaces between cells. This method, however, is related to tissue water content and only indirectly to tissue chemical potential.